Recently, printers and copiers having a higher speed and higher stability have been desired. Moreover, it has been desired to reduce the number of parts because of a demand for further downsizing which is accompanied by higher functions of the parts. In order to obtain a stable density of an image in the electrophotography, a constant developing condition needs to be always provided in a developing process. Whenever the amount of the toner to be charged is unstable, however, a developing bias condition or the like needs to be optimized, for example. Such optimization gives large load to a system for controlling developing properties, often leading to increase in the size of the apparatus or increase in production cost. In order to reduce such load, there has been a demand for improvement in stability of the amount of the toner to be charged, and particularly stability of charging against change in a temperature and humidity.
A variety of proposals for improving environmental stability of the amount of the toner to be charged have been made. Among these, control by a charge control agent prevails, and toners containing a calixarene compound, those using an iron-containing azo dye, and those using an organic boron compound have been proposed (for example, PTLs 1 to 4).